The use of optical radiation for imaging human tissue has many desirable advantages over other imaging methodologies but can require additional complexity in the processing methodology. Although optical radiation can penetrate human tissue reasonably well in the optical and near infrared wavelength regimes, strong scattering in thick tissue results in most of the illuminating light being scattered. As a result, standard tomographic and imaging methodologies provide poor image information. To extract the highest-possible quality in the images, the scattered light must be used or blocked. As early as 1929, light was used to diagnose breast lesions but the scattered light was viewed as noise obscuring the image. The early-arriving photons were viewed as producing the desired high-accuracy two-dimensional tumor localization, while the scattered photons blurred this information. Much later, a similar approach was used to screen for breast cancer. Again, scattered light was viewed as an undesirable artifact. Time-resolved techniques use time-gating to collect the unscattered and minimally-scattered light while screening out the rest of the scattered light. Although these techniques require very short picoseconds to femtoseconds pulses and correspondingly fast detection systems, they have been used to obtain three-dimensional images. Another technique used to discriminate against scattered light is optical coherence tomography which utilizes low coherence interferometry to gate out multiply-scattered light. This method has been used in tissues with thicknesses up to tens of mean scattering lengths.
Another promising category of approaches, called frequency domain methods, utilize diffuse photon density waves (DPDWs) for illumination. These methods explicitly use the scattered light and neglect the unscattered light to image thick tissue. Generally speaking, methods that have been suggested that use DPDWs for illumination are used either for two-dimensional mammography or are tomographic in nature. This usually necessitates movement of the source/detector setup or the tissue samples to obtain depth information, which poses alignment as well as convenience problems when implementing in vivo imaging in a clinical environment. A notable exception is a method which obviates the need for mechanical movement by using multiple sources phased in an appropriate way to scan a spot of high or low photon density throughout the tissue. Thus, a novel method is desired for localizing inhomogeneities in three dimensions employing optical probing radiation to minimize health risks when probing human tissue, and which requires no mechanical movement of the equipment, and that greatly increases the accuracy to which inhomogeneities can be localized. The benefit of such a desired method is that, when used for breast cancer detection, existing mammographic procedures can be used. This can enable quicker acceptance and understanding of the method for use in a clinical environment. Although the optical mammography application of this method is a primary use, this method can be used for localizing inhomogeneities in a wide variety of turbid medium. For example, the invention may be practicable for the detection of pockets of precious commodities such as oil, natural gas and metallic ores. The method of the present invention is an improvement upon previous diffraction tomography systems utilizing a Green's function, whose Fourier transform is purely phase. See U.S. Pat. Nos. 4,594,662, 4,598,366 and 4,562,540 issued to A. J. Devaney and incorporated by reference herein.